ZundEig: The Structure of the Proton in Liquid Water from Unsupervised Learning

SOLANA DI PINO; EDWARD DANQUAH DONKOR; VERÓNICA M. SÁNCHEZ; ALEX RODRIGUEZ ; GIUSEPPE CASSONE; DAMIAN SCHERLIS; ALI HASSALANI

Journal of Physical Chemistry B

American Chemical Society

Año: 2023 vol. 127 p. 9822 - 9832

The structure of the excess proton in liquid waterhas been the subject of lively debate on both experimental andtheoretical fronts for the last century. Fluctuations of the proton aretypically interpreted in terms of limiting states referred to as theEigen and Zundel species. Here, we put these ideas under themicroscope, taking advantage of recent advances in unsupervisedlearning that use local atomic descriptors to characterize environ-ments of acidic water combined with advanced clusteringtechniques. Our agnostic approach leads to the observation ofonly one charged cluster and two neutral ones. We demonstrate thatthe charged cluster involving the excess proton is best seen as anionic topological defect in water’s hydrogen bond network, forminga single local minimum on the global free-energy landscape. Thischarged defect is a highly fluxional moiety, where the idealized Eigen and Zundel species are neither limiting configurations nordistinct thermodynamic states. Instead, the ionic defect enhances the presence of neutral water defects through strong interactionswith the network. We dub the combination of the charged and neutral defect clusters as ZundEig, demonstrating that the fluctuationsbetween these local environments provide a general framework for rationalizing more descriptive notions of the proton in theexisting literature.